Hi,

Please try add more this code to Customize > Custom CSS to do that:

@media only screen and (max-width: 767px){
.container.penci_sidebar .penci-grid > li {
    width: 47% !important;
    margin-right: 6% !important;
}
.container.penci_sidebar .penci-grid > li:nth-child(2n+2){
    margin-right: 0 !important;
}
.container.penci_sidebar .penci-grid > li:nth-child(2n+1){
    clear: both !important;
}
}

Best Regards,

PenciDesign